Three-minute peripheral blood film evaluation: The erythron and thrombon

Evaluating a peripheral blood film validates cell counts performed by hematology analyzers, plus it offers valuable diagnostic information relayed by erythrocytes, platelets, and leukocytes. The first article in this symposium described how to prepare a peripheral blood film. In this article, we discuss important red blood cell (RBC) and platelet number and morphologic changes. And in the next article, we discuss white blood cell (WBC) alterations.

If a patient's RBC mass is reduced, then the patient is anemic. Once anemia is recognized, the next concern is bone marrow responsiveness: Is the anemia regenerative or nonregenerative? If the RBC bone marrow precursor cells respond with increased production of appropriate magnitude, the anemia is regenerative (responsive). If RBC production is not effectively increased, the anemia is nonregenerative (nonresponsive). Keep in mind that a lag of 48 to 72 hours exists before bone marrow responsiveness is perceived in the peripheral blood, so be careful in defining nonresponsiveness.

Evaluating the thrombon by first assessing platelet numbers is another important part of every complete blood count (CBC). Thrombocytopenia is more common than thrombocytosis and can be of great clinical importance. Platelet counts below 40,000/µl can lead to spontaneous bleeding. Platelet clumping, which can be seen in any species but is particularly problematic in cats, can give a falsely low platelet count. Platelet clumping frequently interferes with results from impedance cell counters because aggregated platelets may be included in the RBC or WBC counts, resulting in artifactually decreased platelet counts.1

In dogs and cats, polychromasia is the principal feature of RBC regeneration on a blood film prepared with Wright's or modified Wright's stain. Polychromatophils are immature RBCs that stain bluish because they contain RNA (Figure 1). Polychromatophils on blood smears prepared with Wright's or modified Wright's stain roughly correspond to reticulocytes on smears prepared with new methylene blue stain. If regeneration is still questionable after you've evaluated Wright's-stained blood films, perform a reticulocyte count with new methylene blue stain.

When performing reticulocyte counts in cats, it is important to count aggregate reticulocytes (cells containing diffuse accumulations of reticulum) since they represent recent RBC production in response to relatively severe anemia (Figure 2). In cats, aggregate reticulocytes mature into punctuate reticulocytes within 12 hours, and punctuate reticulocytes mature into RBCs in about 10 days. Consequently, elevated punctuate reticulocytes represent RBC regeneration two weeks earlier, whereas aggregate reticulocytes indicate recent regeneration.2 In dogs and cats, peak reticulocyte counts occur four to eight days after the onset of anemia.

As a general guideline, a reticulocyte count above 60,000/µl in cats (aggregate only) and 80,000/µl in dogs indicates a regenerative anemia.3 Dogs with regenerative anemias have reticulocyte counts that are frequently 100,000 to 300,000/µl (dogs with extremely regenerative anemia can have counts of 500,000/µl or greater), while cats typically demonstrate a less dramatic response when only the aggregate reticulocyte counts are evaluated.

Reference laboratories can provide reticulocyte counts, and in some cases, they will automatically add the reticulocyte count to the CBC at an additional charge if anemia is identified. The newer in-clinic, laser-based hematology analyzers automatically provide absolute reticulocyte counts with each CBC.4